Optionally de-vendor libmdbx

[SamWilsn]

When packaging for Linux distributions (in my case, Gentoo), it's generally the policy to link against the system-wide installed versions of a library rather than bundling it. The pattern I've used in this PR is similar to what's used by openssl-sys and other crates.

[CLAassistant]

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[SamWilsn]

May I ask why you've chosen to close this?

[vorot93]

a) You've not signed the CLA. b) Wrapping a different version of libmdbx than the one vendored is something to be avoided. The only way to ensure this is having libmdbx vendored.

[SamWilsn]

I'll sign the CLA if it's required to get this merged, but there's no point if you won't accept the patch.

If the dependency on mdbx is really that strict, I can specify an exact version in the packaging script. The only people who use this feature will be distro maintainers, and hopefully they can be expected to understand its impact.